1. Field of the Invention
The present invention relates to a phase shift mask blank, a phase shift mask, and a method of manufacturing phase shift masks using a novel phase shifter material.
2. Prior Art
The shifter material used in halftone phase shift masks is often composed primarily of molybdenum silicide, although chromium oxide-based materials are also used.
As shown in FIGS. 10A and 10B, which are incorporated in and constitute part of the specification, a halftone phase shift mask is comprised of a quartz substrate 32 on which there is provided a shifter 34 which changes the phase of the light. The mask improves resolution by utilizing an interference effect between light that passes through the shifter 34 and undergoes a change of phase, and light that does not pass through the shifter 34 and does not undergo a change of phase.
The trends toward a higher level of integration and higher processing speed in large-scale integration (LSI) chips have created a need for a smaller pattern rule in semiconductor devices. The photomasks used to form those finer patterns must likewise be produced to a smaller feature size.
Efforts are being made to develop phase shift masks which meet these criteria. However, further reduction in the minimum feature size on the masks will require changing the exposure wavelength of light emitted by the light source used during mask fabrication from the i-line wavelength (365 nm) to that of KrF excimer laser light (248 nm), ArF excimer laser light (193 nm), and eventually F2 laser light (157 nm).
This is because, in lithography, the resolution is proportional to the wavelength of the exposure light, as indicated by the Rayleigh formula:
R=kxcex/NA 
In the formula, R is the resolution, k is the process coefficient, xcex is the wavelength, and NA is the numerical aperture of the lens.
However, the molybdenum silicide-based shifter films which are most commonly used today have such a large absorption coefficient that very little, if any, short-wavelength light in the ArF excimer laser light (193 nm) and F2 laser light (157 nm) regions passes through. Hence, these films are unsuitable for use together with such shorter wavelength exposure light sources.
In the case of chromium-based shifter films, those composed of chromium metal only have very little transmittance for the simple fact that they are metal. Even if the chromium metal has oxygen, nitrogen or carbon added to it, a transmittance sufficient for use as a phase shifter material in a short-wavelength region of 193 nm or less, such as a transmittance of 3 to 40%, has been difficult to achieve.
Moreover, because short-wavelength light of 193 nm or less has a much higher energy than 365 nm or 248 nm light, like the mask substrate and the lens optics, the phase transfer material is subject to deterioration over time. A need is thus felt for the development of a material capable of enduring high-energy irradiation.
At the same time, the phase shifter material must be capable of causing a 180 degree shift in the phase of light that passes through the shifter layer relative to light that does not pass through. Bearing in mind the topography of the shifter layer pattern, by forming the shifter film to a thickness D, defined as
D=xcex/2(nxe2x88x921) xe2x80x83xe2x80x83(1) 
from a material having a high refractive index n, a 180-degree phase shift can be achieved at a small film thickness or step on the substrate. In the foregoing formula, D is the shifter film thickness for generating a 180-degree phase shift, n is the refractive index of the shifter material, and xcex is the wavelength of the transmitted light.
However, prior-art chromium-based and molybdenum silicide-based shifter materials cannot provide a high refractive index at shorter exposure light wavelengths (i.e., wavelengths of 193 nm or less), and thus must have a large film thickness, making it difficult to achieve a phase shift of 180 degrees.
It is, therefore, an object of the present invention to provide phase shift mask blanks and phase shift masks which resolve the above drawbacks of prior-art halftone phase shift masks and make it possible to fabricate semiconductor integrated circuits having a smaller minimum feature size and a higher degree of integration. Another object of the invention is to provide a method of manufacturing such phase shift masks.
It has been found that phase shift mask blanks and phase shift masks comprising in part a phase shifter composed primarily of fluorine-doped metal silicide, and especially a phase shifter composed primarily of fluorine-doped chromium silicide, fluorine-doped molybdenum silicide or fluorine-doped gadolinium gallium silicide, have a good transmittance of from 3 to 40% to short-wavelength light of 193 nm or less, such as ArF excimer laser light and F2 laser light, that cannot be attained with the chromium-based and molybdenum silicide-based shifter materials used to date. Such phase shift masks also have excellent durability to high energy irradiation and improved stability over time. It has also been found that fluorine-doped metal silicide films, by virtue of their high refractive indices, are capable of providing a 180-degree phase shift in transmitted light at a relatively small film thickness, thereby making it possible to minimize the effects (e.g., primarily focal depth) of shifter film thickness on light exposure. These discoveries have made it possible to effectively resolve the problems inherent in prior-art halftone phase shift masks, enabling phase shift masks to be provided which are capable of further reducing the minimum feature size and increasing the level of integration in semiconductor integrated circuits.
Accordingly, in a first aspect, the invention provides a phase shift mask blank comprising a transparent substrate and at least one layer of phase shifter on the substrate, wherein the phase shifter is a film composed primarily of a fluorine-doped metal silicide.
In a second aspect, the invention provides a phase shift mask blank comprising a transparent substrate, at least one layer of phase shifter on the substrate, and at least one layer of chromium-based film on the phase shifter, wherein the phase shifter is a film composed primarily of a fluorine-doped metal silicide. The chromium-based film is preferably a light-shielding film, an antireflection coating or a multilayer combination of both. More specifically, the chromium-based film is a CrC film, CrCO film, CrCN film or CrCON film, or a multilayer combination thereof.
In the phase shift mask blank of the first or second aspect of the invention, the fluorine-doped metal silicide is preferably fluorine-doped chromium silicide, fluorine-doped molybdenum silicide or fluorine-doped gadolinium gallium silicide, and typically contains one or more element selected from among oxygen, nitrogen and carbon. Preferably, the phase shifter composed primarily of a fluorine-doped metal silicide shifts the phase of exposure light passing through it by 180xc2x15 degrees and has a transmittance of 3 to 40%.
In a third aspect, the invention provides a phase shift mask produced by patterning the phase shifter on the phase shift mask blank according to the first or second aspect of the invention.
In a fourth aspect, the invention provides a method of manufacturing a phase shift mask, comprising the steps of forming a phase shifter composed primarily of a fluorine-doped metal silicide on a substrate transparent to exposure light using a sputtering technique, lithographically forming a resist pattern on the phase shifter, and patterning the phase shifter by dry etching or wet etching through the resist pattern.
In a fifth aspect, the invention provides a method of manufacturing a phase shift mask, comprising the steps of forming a phase shifter composed primarily of a fluorine-doped metal silicide on a substrate transparent to exposure light using a sputtering technique, forming a chromium-based film on the phase shifter using a sputtering technique, removing areas of the chromium-based film where exposure to light is necessary by etching so as to leave corresponding areas of the phase shifter exposed on the surface, lithographically forming a resist pattern on the phase shifter, and patterning the phase shifter by dry etching or wet etching through the resist pattern.
In the phase shift mask manufacturing method according to the fourth and fifth aspects of the invention, the fluorine-doped metal silicide is preferably fluorine-doped chromium silicide, fluorine-doped molybdenum silicide or fluorine-doped gadolinium gallium silicide. In the step of forming a phase shifter, sputtering is preferably carried out using chromium, molybdenum or gadolinium gallium as the target, and using SiF4 as the reactive gas; or using chromium silicide, molybdenum silicide or gadolinium gallium silicide as the target, and using SiF4, CF4 or NF3 as the reactive gas.
In the inventive method, sputtering is typically carried out by reactive sputtering using a mixed gas composed of an element source gas which supplies an element selected from among oxygen, nitrogen and carbon in admixture with an inert gas and the reactive gas. Preferably, the element source gas is used at a flow rate such that the elemental ratio of the element thus supplied relative to the inert gas is 1 to 40% for oxygen, 1 to 20% for nitrogen, and 1 to 10% for carbon. In the manufacturing method of the invention, it is preferable for the phase shifter composed primarily of fluorine-doped metal silicide to shift the phase of exposure light passing through it by 180xc2x15 degrees and to have a transmittance of 3 to 40%.